Compliant annulus for loudspeaker and related circuit



R. T. BOZAK April 1, 1969 COMPLIANT ANNULUS FOR LOUDSPEAKER AND RELATED CIRCUIT Sheet Filed Oct. 11, 1965 TIME FIG. 2

INVENTOR. RUDOLPH T. BOZAK BY 9 $91.42

ATTORNEYS APril 1969 R. 1'. BOZAK 3,436,494

COMPLIANT ANNULUS FOR LOUDSPEAKER AND RELATED CIRCUIT Filed on. 11. 1965 Sheet 2 oi 2 44 INVENTOR 34C RUDOLPH TI BOZAK FIG. 6 BY 7/ 4, WM 41w,

ATTORNEYS United States Patent US. Cl. 179115.5 11 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a speaker structure constructed to give linear response at low frequencies. This is accom plished by first providing a balance bridge circuit having a DC element therein which, upon application of a predetermined direct current, buoys up the speaker diaphragm to counterbalance and overcome sagging of the diaphragm. This buoying effect also centers the diaphragm along the speaker acoustical axis. Secondly, the diaphragm is provided with a compliant annulus support peripherally relieved to provide a plurality of leaf springs which provide excellent lateral restraining action upon the diaphragm and which increase the degree of compliance to give the desired linearity of diaphragm displacement.

The present invention relates to an improvement in a permanent magnet dyanmic type loudspeaker and particularly to extended low frequency, low distortion loudspeakers which can reproduce sounds in the sonic and subsonic ranges.

Permanent magnet dynamic loudspeakers or movablecoil loudspeakers as they are often designated in the art operate on the theory that a speaker diaphragm, when attached to a moving voice coil which is carrying a signal and is located in the field of a permanent magnet, will vibrate axially in response to the interaction of the magnetic fields of the coil and the permanent magnet. Sound Waves of a frequency related to the current in the coil are thereby radiated by the diaphragm into the surrounding air.

Difiicult engineering problems confronted the loudspeaker manufacturers when they attempted to maintain true fidelity in dynamic speakers which were designed to operate in low frequency ranges. Very low frequency response, in the range below 50 c.p.s., dictates the use of a diaphragm of increased diameter and a highly compliant annulus suspending the diaphragm from the edge of the speaker housing. Enlargement of the diaphragm however increases its mass and causes the diaphragm and its associated voice coil to sag below the initial rest position. Use of a highly compliant annulus will only magnify the problem with an overall result of distortion of the low frequency sound reproductions.

The ideal annulus should possess very high compliance, with little resistance to the movement of the speaker diaphragm and yet should be stiff enough to provide a restoring force which very quickly returns the speaker diaphragm to its initial position. Edge suspensions in conventional loudspeaker design do not lend themselves to use in extended low frequency, low distortion response applications. First of all, if the compliance is made sufiiciently high, it has no lateral restraining action, thus axial misalignment in the voice coil gap will occur. Second, in the conventional corrugated or bead type of edge the corrugations do not roll in both directions with equal ease and this results in nonlinear compliance and distortion. Third, if compliance problems are offset by increasing mass of the diaphragm so as to attain low resonance, transient response is sacrificed, dampening is impaired and efficiency is reduced. Fourth, when a highly compilant annulus is used there is no restoring in the system and it actually sags.

A further disadvantage of a conventional speaker system is due to the asymmetrical distribution of the magnetic field of its permanent magnet. The irregularly shaped field of the permanent magnet will interact with the magnetic field generated by the AC signal in such a manner as to cause nonlinear excursions of the diaphragm and voice coil. Since the configuration of the magnetic field varies with each permanent magnet, the same AC signal in two otherwise identical speaker systems potentailly can effect different axial displacements.

The present invention overcomes these difiiculties. The tendency of the diaphragm to sag is controlled by means of a balance bridge circuit connected to the geometric center of the voice coil. The balance bridge circuit contains a DC element which tends to make the diaphragm weightless. Preferably a variable resistive means is employed for directing current from a DC source into the desired section of the coil. Therefore a diaphram that is sagging because of excessive mass can be buoyed up by increasing the DC element in the upper section of the coil. Furthermore, this DC biasing materially improves the dynamic linearity by minimizing the effect of the asymmetrical distribution of the flux in the gap of the permanent magnet.

The balance bridge is most effective when utilized in connection with an annulus exhibiting high compliance. In accordance with the present invention, a highly compliant annulus has been especially devised to cooperate with the balance bridge circuit to produce the highest fidelity in low frequency ranges. The compliant annulus is preferably made of Bakelite so cut out as to provide leaf springs which display the requisite high compliance and the necessary linearity of displacement. The leaf springs are in general arranged circumferentially and as a result, the width of the area occupied by the springs is not more than about one-fourth of the radius of the annulus. Another advantage of the circumferential leaf spring arrangement is that the total linear displacement of the speaker diaphragm may be up to about one inch and yet there is axial stability. Since there is axial stability the speaker diaphragm and voice coil vibrate along the acoustical axis of the unit and the voice coil does not scrape against the permanent magnet. The spring elements are preferably spaced equally around the periphery of the .diaphragm in a symmetrical manner to prevent the diaphragm from tipping as it moves on the acoustical axis. The same results can be obtained with a nonsymmetrical arrangement of the leaf springs but in such case the Width or thickness would be varied around the periphery to compensate for the nonsymmetrical arrangement. A further advantage of the present construction is that it is a very simple matter to change the length, width or thickness of the leaf springs and thereby obtain any desired compliance and restoring force.

The openings in the Bakelite annulus are preferably sealed so that the diaphragm will radiate a maximum volume of sound. This is best accomplished by applying a plastic or rubberized thin skin to the underside of the compliant annulus. For very large displacements, an excess of loose material is employed to permit displacement without fracturing the membrane. An important feature of the leaf spring construction is that a speaker unit employing the new annulus can be positioned on any angle without losing linear stability.

Other advantages of the invention will become apparent in the following description of preferred embodiments of the invention selected for the purpose of illustrating the principle of the invention.

For ease of description reference will be made to the accompanying drawings in which:

FIG. 1 is a sectional view of a conventional loudspeaker assembly. 7

FIG. 2 is a graph of two curves comparing the linear operation of a speaker when the 'balance bridge circuit is employed and the nonlinear response when it is not used.

FIG. 3 is a circuit plan for superimposing the direct current on to the AC signal, which centers the diaphragm and tends to make it weightless.

FIGS. 4 and 5 illustrate modifications of the DC circuit of the invention.

FIG. 6 is a top plan view of the new compliant annulus.

FIG. 1 shows the usual convention rigid loudspeaker housing 2 which has a permanently magnetized pole piece 4 and a core 6 with a space for receiving the voice coil 8 of a cone-shaped diaphragm 10. The voice coil has electrically conductive wires 12 wound around the exterior surface thereof. The pole piece 4 and the core 6- provide a magnetic flux in the voice coil aperture 14 and AC electric signals conducted through wires 12 generate a second magnetic flux. The interaction of the two magnetic fluxes induces axial movement of the voice coil to which the diaphragm is attached. Diaphragm 10 is supported by a compliant annulus 13- at the top and bottom. A rubber dust cover is indicated at 15.

A typical graph of the linear displacement of the diaphragm of FIG. 1 is illustrated in FIG. 2 by the dotted line 16. Due to the sag and asymmetrical distribution of flux in the system, the dotted line has a nonlinear portion at 18.

In accordance with the present invention, the circuit of FIG. 3 is connected to the voice coil 8 of diaphragm 10 in order to bias the diaphragm and tend to make it weightless. Direct current from a suitable source 19 is fed into the voice coil at a point between the ends thereof and preferably in the center as at 20 where the flow of current divides to flow into the top and bottom halves of the coil. A variable resistance 22 is preferably employed for adjusting the flow of current to the coil to achieve optimum operating characteristics for the particular unit at hand. For example, with a direct current of 6 volts and a variable resistance of 200 ohms, the dotted line curve of the unit of FIG. 1 was shifted to the solid line 24 which as can be seen very nearly approaches a perfect linearity curve for optimum fidelity in the reproduction of the input signal. The input signal generated at 26 is fed through the voice coil in conventional manner at 28 and 30. In order to avoid shunting of the input signal the resistance of 22 must be made substantially greater than the resistance of the voice coil 8.

FIG. 4 illustrates a modified form of circuit for feeding direct current to the voice coil in accordance with the present invention. As shown in FIG. 4, two variable resistors 50 and 51 are used, each located in an outer branch of the circuit. A center tap 20 is again employed in the middle branch of the circuit which also contains the DC voltage source 19.

FIG. 5 illustrates another way in which the direct current may be fed into the voice coil in accordance with the present invention. The circuit is similar to that of FIG. 4 except that the leg of the circuit containing the resistor 50 is eliminated and variable resistor 51 is the fixed resistance 52. The DC voltage from source 19 is then impressed across the top of the coil through resistance 52 thereby reducing sag of the voice coil and diaphragm. In this embodiment the position of the tap 20 may be varied.

When the diaphragm 10 has no effective mass due to the lifting effect of the balancing circuit of FIGS. 3 and 5, it will readily adapt to use in low frequency, low distortion applications down in the subsonic ranges. However, in low frequency design the diaphragm of the speaker shown in FIG. 1 must be suspended from its supporting edge 31 with a high degree of compliance capable of total linear displacements of up to about one inch.

To this end the compliant annulus 32 is made from a 4 flat sheet of Bakelite or other suitable material which is provided with cutouts 34 in order to form a plurality of solid leaf springs 36 which are in general positioned in a circumferential arrangement. Each separate cutout comprises two generally circumferential slits 34a and 34b positioned in radial spaced relationship and the slits 34a and 34b are joined by a third shorter slit 34c which bridges across the width of the annulus. As shown in the drawing the length of the short slit 340 is about one fifth the length of a circumferential slit and the short slit runs generally in a radial direction. For best results the short slit 340 should not be more than about one fifth the length of the long slits 34a and 34b. Each separate cutout is in general in the form of an elongated Z.

For best compliance the circumferential slits of adjacent cutouts project into a common area of the annulus to provide areas in which two of the circumferential slits cut across a common radius across the width of the compliant annulus.

The width of the annulus at any one side is not more than about one fourth of the radius to the outer periphery. For best results of stability in operation the inner end 38 of each leaf spring is connected to the inner ends of all of the other leaf springs by means of a continuous narrow strip 40 which delineates the inside periphery of the annulus. In like manner, the outer end of each leaf spring 42 is connected to the outer ends of all of the other leaf springs by means of a continuous strip 44 which forms the outside periphery of the annulus. If desired the inside or outside strip may be omitted for an exceptionally high compliance but in such case the stability will be materially lower. Alternatively some of the leaf springs may be disconnected from the inside or outside strip to increase the compliance. Six leaf springs are employed in the structure of FIG. 6 but the number may be varied to suit the desired compliance in the unit at hand.

The compliance annulus 32 is attached to the speaker diaphragm 10 in conventional manner and a thin rubber annulus 46 or other highly flexible material is attached across the width of the annulus to form an air seal for best results. The excess material in the loop 48 enables the annulus 46 to freely follow the excursions of the speaker diaphragm without breaking the air seal. It will be understood that the compliant annulus 32. may be used to advantage in some installations without using the DC circuit of the present invention for biasing the speaker diaphragm 10.

This invention is not to be limited to the details of construction and arrangement set forth herein, as it will be obvious to those skilled in the art that 'various modifications may be made without departing from the spirit and scope of the invention.

What I claim is:

1. In a loudspeaker of the type which comprises a speaker diaphragm with voice coil positioned in the magnetic field of a permanent magnet to generate sonic or subsonic vibrations in response to AC electrical signals flowing in the coil, the improvement which comprises a source of direct current, and a circuit for feeding direct current from said source through the speaker coil, said circuit including means fo rcontrolling the magnetic field generated by said direct current so that said magnetic field substantially counterbalances sagging of the diaphragm, thereby causing said diaphragm to substantially align with the acoustical axis of said speaker.

2. A structure as specified in claim 1 which includes at least one compliant annulus for supporting the speaker diaphragm, said compliant annulus having a plurality of circumferential cutouts that form leaf springs within the annulus.

3. A structure as specified in claim 1 in which the direct current is fed into the voice coil at a point spaced away from the ends thereof to provide a divided flow of current through the coil.

4. A structure as specified in claim 1 in which the DC circuit includes a variable resistance for controlling the flow of current through the coil.

5. In a structure of the type which comprises a diaphragm with voice coil positioned in the magnetic field of a permanent magnet to generate vibrations in response to AC electrical signals flowing in the coil, the improvement which comprises a source of direct current, and a circuit for feeding direct current from said source through the coil, said circuit including means for controlling the magnetic field generated by said direct current so that said magnetic field substantially counterbalances sagging of said diaphragm, thereby causing said diaphragm to substantially align with the acoustical axis of said speaker.

6. In a loudspeaker of the type which comprises a speaker diaphragm with voice coil positioned in the magnetic field of a permanent magnet to generate sonic or subsonic vibrations in response to AC electrical signals flowing in the coil, the improvement which comprises a compliant annulus having a plurality of cutouts therein which form circumferential leaf springs within the annulus.

7. A structure as specified in claim 6 in which the inner ends of a plurality of said leaf springs are connected to a continuous strip which forms the inner periphery of said annulus and in which the outer ends of a plurality of said leaf springs are connected to a continuous strip which forms the outer periphery of said annulus.

8. A structure as specified in claim '6 in which the cutouts are generally Z-shaped and said annulus is made of thin fiat sheet material.

9. A structure as specified in claim 6 in which each separate cutout comprises two circumferential slits positioned in radial spaced relationship and joined by a third shorter slit positioned to bridge across the width of the annulus between the two radially spaced circumferential slits.

10. A structure as specified in claim 9 in which the circumferential slits of adjacent separate cutouts project into a common area in which two of said circumferential slits cross a single line of radius across the width of the annulus.

11. A structure as specified in claim 6 in which the width of the annulus at any one side is not more than one fourth of the radius.

References Cited 

